This invention relates in general to electrophotographic imaging members and more specifically, to a process and apparatus for fabricating the electrophotographic imaging members.
In the art of xerography, a xerographic plate containing a photoconductive insulating layer is imaged by first uniformly electrostatically charging its surface. The plate is then exposed to a pattern of activating electromagnetic radiation such as light, which selectively dissipates the charge in the illuminated areas of the photoconductive insulator while leaving behind an electrostatic latent image in the non-illuminated areas. This electrostatic latent image may then be developed to form a visible image by depositing finely divided electroscopic marking particles on the surface of the photoconductive insulating layer.
A photoconductive layer for use in xerography may be a homogeneous layer of a single material such as vitreous selenium or it may be a composite layer containing a photoconductor and another material. One type of composite photoconductive layer used in xerography is illustrated in U.S. Pat. No. 4,265,990 in which a photosensitive member having at least two electrically operative layers is described. One layer comprises a photoconductive layer which is capable of photogenerating holes and injecting the photogenerated holes into a contiguous charge transport layer. Generally, where the two electrically operative layers are supported on a conductive layer with the photoconductive layer capable of photogenerating holes and injecting photogenerated holes sandwiched between the contiguous charge transport layer and the supporting conductive layer, the outer surface of the charge transport layer is normally charged with a uniform charge of a negative polarity and the supporting electrode is utilized as an anode. Obviously, the supporting electrode may also function as an anode when the charge transport layer is sandwiched between the electrode and a photoconductive layer which is capable of photogenerating electrons and injecting the photogenerated electrons into the charge transport layer. The charge transport layer in this embodiment, of course, must be capable of supporting the injection of photogenerated electrons from the photoconductive layer and transporting the electrons through the charge transport layer.
Various combinations of materials for charge generating layers and charge transport layers have been investigated. For example, the photosensitive member described in U.S. Pat. No. 4,265,990 utilizes a charge generating layer in contiguous contact with a charge transport layer comprising a polycarbonate resin and one or more of certain aromatic amine compound. Various generating layers comprising photoconductive layers exhibiting the capability of photogeneration of holes and injection of the holes into a charge transport layer have also been investigated. Typical photoconductive materials utilized in the generating layer include amorphous selenium, trigonal selenium, and selenium alloys such as selenium-tellurium, selenium-tellurium-arsenic, selenium-arsenic, and mixtures thereof. The charge generation layer may comprise a homogeneous photoconductive material or particulate photoconductive material dispersed in a binder. Other examples of homogeneous and binder charge generation layer are disclosed in U.S. Pat. No. 4,265,990. Additional examples of binder materials such as poly(hydroxyether) resins are taught in U.S. Pat. No. 4,439,507. The disclosures of the aforesaid U.S. Pat. No. 4,265,990 and U.S. Pat. No. 4,439,507 are incorporated herein in their entirety. Photosensitive members having at least two electrically operative layers as disclosed above in, for example, U.S. Pat. No. 4,265,990 provide excellent images when charged with a uniform negative electrostatic charge, exposed to a light image and thereafter developed with finely developed electroscopic marking particles. However, when the charge transport layer comprises a film forming resin and one or more of certain diamine compound, difficulties have been encountered with these photosensitive members when they are used in high volume, high speed copiers, duplicators and printers. For example, it has been found that when certain charge transport layers comprise a film forming resin and an aromatic amine compound, the dark decay characteristics are unpredictable from one production batch to another. Dark decay (V.sub.DDP) is defined as the loss of charge on a photoreceptor in the dark after uniform charging. This unpredictability characteristic is highly undesirable, particularly for high volume, high speed copiers, duplicators and printers which require precise, stable, and predictable photoreceptor operating ranges. Erratic variations in dark decay rate can be unacceptable or at, the very least, require expensive and sophisticated control systems or trained repair persons to alter machine operating parameters such as charging potentials, toner concentration and the like to compensate for different photoreceptor dark decay rates. Failure to adequately compensate for dark decay rate differences when replacing photoreceptors in a machine can result in copies of poor copy quality. Moreover, such variations in dark decay rate prevent achievement of optimized dark decay properties.
Similarly, photoreceptors utilizing charge transport layers comprising a film forming resin and one or more of certain aromatic amine compounds also exhibit erratic variations in background potential (V.sub.BG) from one production batch to another. Background potential is defined as the potential in the background or light struck areas of a photosensitive member after exposure to a pattern of activating electromagnetic radiation such as light. Unpredictable variations in background potential can adversely affect copy quality, especially in complex, high volume, high speed copiers, duplicators and printers which by their very nature require photoreceptor properties to meet precise narrow operating windows. Thus, like photoreceptors that exhibit batch to batch dark decay variations, photosensitive members that have poor background potential characteristics are also unacceptable or require expensive and sophisticated control systems or trained repair persons to alter machine operating parameters. Inadequate compensation of background potential variations can cause copies to appear too light or too dark. In addition, such variations in background potential properties preclude optimization of background potential properties.
Control of both V.sub.DDP and V.sub.BG of photosensitive members is important not only initially but through the entire cycling life of the photosensitive members.
Thus, the characteristics of photosensitive members comprising a conductive layer and at least two electrically operative layers, one of which is a charge transport layer comprising a film forming resin and one or more aromatic amine compounds, exhibit deficiencies which are undesirable in high quality, high volume, high speed copiers, duplicators, and printers.
One technique to overcome the above described deficiencies was the development of a process for preparing an electrophotographic imaging member in which a photogenerating layer on a supporting substrate was coated with a charge transport layer forming mixture, the charge transport layer forming mixture comprising an aromatic amine compound of one or more compounds having the general formula: ##STR1## wherein R.sub.1 and R.sub.2 are an aromatic group selected from the group consisting of a substituted or unsubstituted phenyl group, naphthyl group, and polyphenyl group and R.sub.3 is selected from the group consisting of a substituted or unsubstituted aryl group, alkyl group having from 1 to 18 carbon atoms and cycloaliphatic compounds having from 3 to 18 carbon atoms, a polymeric film forming resin in which the aromatic amine is soluble, a solvent such as methylene chloride for the polymeric film forming resin, and from about 1 part per million to about 10,000 parts per million, based on the weight of the aromatic amine compound, of a protonic acid or Lewis acid having a boiling point preferably greater than about 40.degree. C. and soluble in the solvent. This process is disclosed in U.S. Pat. No. 4,725,518 to Kathleen M. Carmichael et al on Feb. 16, 1988, the entire disclosure of this patent being incorporated herein by reference. Although excellent results are achieved with the process of the invention described in U.S. Pat. No. 4,725,518, difficulties have been encountered in continuous manufacturing processes where long lengths of webs are coated at high speeds. More specifically, when a large production run of a multilayered photoreceptor is prepared by coating, most of the layers are applied sequentially in a continuous process which can include the application, for example, of a charge generating layer followed by drying the layer and the subsequent application of charge transport layer followed by further drying. Since the electrical properties of the final coated photoreceptor web can vary along its length during the coating operation due to various factors such as compositional changes in the charge generating layer coating mixture and the charge transport layer coating mixture as these coating mixtures are replenished during long coating runs, the electrical properties of the photoreceptor are tested after drying of the final coating while the coating run is in progress. Thus, if the electrical properties are unsatisfactory due to effects caused by changes in the electrical properties of the deposited and dried charge generating layer, the composition of the charge transport layer must be changed to compensate for the changes in the charge generating layer to achieve the desired electrical properties in the final photoreceptor product. Further, as the charge transport coating materials are replenished during a coating run, variations in the electrical properties along the length of the final dried transport layer can also affect the electrical properties of the final photoreceptor product. Since the charge transport layer is normally the last layer to be applied to a multilayered photoreceptor, alterations of the chemical composition of the charge transport layer immediately prior to coating can provide some control during the coating process to achieve the desired uniformity of electrophotographic performance notwithstanding variations in the electrical properties of the charge generating layer as it is being applied. Thus, if the electrical performance of a coated web after the transport layer emerges from a drying station is unsatisfactory, the entire coating line must be stopped and the concentration of the protonic acid or Lewis acid additive in the charge transport layer coating composition altered, for example, by adding additional additive and stirring it into the coating solution. Unfortunately, substantial time is required to achieve a uniform mixture of the additive dispersed throughout the charge transport solution. This is partly due to the high viscosity of the charge transport solution, e.g. 800 cp, and the large volume of coating solution utilized compared relative to the extremely small amount of additive added. Nonuniform mixtures cause still more variations in the electrical properties of the final photoreceptor product. Thus, the entire coating line may be shut down for as long as, for example, two hours while the additive is stirred into the viscous charge transport coating mixture. Moreover, in large production runs where the distances between the supply roll for the web and the location of the testing station downstream of the charge transport layer drying station losses can be very high, e.g., as much 600 feet of coated web have had to be scrapped. Moreover, in order to start up the line again and attain proper coating speed and to adjust the coating stations to achieve the desired mixture concentrations and uniformity, as much as an additional 1000 feet of coated web may be lost. This large loss of material involves not only the web substrate, but also all the other materials applied to the substrate. In addition, it has been found to be very difficult to determine the exact amount of additive to be added to achieve the desired final electrical properties because of the very small amounts utilized, e.g. 20 ppm, of the additive. Also, as described previously, alterations of the composition of the generator layer coating material as it is replenished can require further changes to the composition of the charge transport layer. Thus, for practical reasons, wide variations in the electrical properties of the final photoreceptor must be accepted. These wide variations in the electrical properties require complex designs in the charging, exposure, and development stations in modern, sophisticated high speed copiers, duplicators and printers to compensate for these variations so that high quality images can be produced.
Although the charge transport layer could be formulated by mixing metered amounts of material from two different sources of charge transporting coating compositions having two different concentrations of additive, mixing of the two compositions would still require considerable time which affects the amount of unacceptable coated photoreceptor material fabricated on the fly during the mixing process. Also, if mixing is accomplished during long runs, large amounts of material are also lost during cleaning of the mixing system.